Apparatus and method for controlling an electric machine

ABSTRACT

Embodiments of the present invention provide An electric machine control system for a vehicle, the electric machine control system comprising one or more controllers, wherein the vehicle comprises an electric machine arranged to be selectively coupleable to provide torque to at least one wheel of an axle of the vehicle, the control system comprising input means ( 330 ) to receive at least one attribute signal ( 410 - 4 X 0 ) indicative of one or more attributes of the vehicle ( 100 ), output means ( 340 ) arranged to output a coupling signal ( 345 ) to control coupling of the electric machine to the at least one wheel of the axle ( 222 ), and processing means ( 310 ) arranged to determine ( 1310 ) a coupling state of the electric machine ( 212 ) to the at least one wheel of the axle in dependence on the at least one attribute signal, wherein the processing means is arranged to control the output means to output the coupling signal ( 345 ) indicative of the determined coupling state, wherein the processing means is arranged to defer ( 1320, 1330 ) controlling the output means to output the coupling signal indicative of a change in the coupling state until expiry of a predetermined period of time since a last change in the coupling state indicated by the coupling signal.

TECHNICAL FIELD

The present disclosure relates to controlling an electric machine andparticularly, but not exclusively, to controlling coupling of anelectric machine. Aspects of the invention relate to a control system,to a powertrain, to a vehicle, to a method and to computer software.

BACKGROUND

It is increasingly known for vehicles to be powered by more than onemotive or traction power source, such as an internal combustion engineand one or more electric machines or motors. However, management ofmultiple traction power sources may be problematic.

It is an object of embodiments of the invention to at least mitigate oneor more of the problems of the prior art.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide a control system, apowertrain, a vehicle, a method and computer software as claimed in theappended claims.

According to an aspect of the invention, there is provided an electricmachine control system for a vehicle, the electric machine controlsystem comprising one or more controllers, wherein the vehicle comprisesan electric machine arranged to be selectively coupleable to providetorque to at least one wheel of an axle of the vehicle processing meansarranged to determine a coupling state of the electric machine to the atleast one wheel of the axle. Advantageously the processing means isarranged to determine the coupling of the electric machine to the atleast one wheel of the axle.

According to an aspect of the invention, there is provided an electricmachine control system for a vehicle, the electric machine controlsystem comprising one or more controllers, wherein the vehicle comprisesan electric machine arranged to be selectively coupleable to providetorque to at least one wheel of an axle of the vehicle, the controlsystem comprising: input means to receive at least one attribute signalindicative of one or more attributes of the vehicle; output meansarranged to output a coupling signal to control coupling of the electricmachine to the at least one wheel of the axle; and processing meansarranged to determine a coupling state of the electric machine to the atleast one wheel of the axle in dependence on the at least one attributesignal, wherein the processing means is arranged to control the outputmeans to output the coupling signal indicative of the determinedcoupling state at lower than a predetermined frequency indicative of achange in coupling state. The predetermined frequency is determined independence on a predetermined period of time since a last change in thecoupling state. Advantageously excessively frequent changes in couplingstate are prevented.

According to an aspect of the invention, there is provided an electricmachine control system for a vehicle, the electric machine controlsystem comprising one or more controllers, wherein the vehicle comprisesan electric machine arranged to be selectively coupleable to providetorque to at least one wheel of an axle of the vehicle, the controlsystem comprising: input means to receive at least one attribute signalindicative of one or more attributes of the vehicle; output meansarranged to output a coupling signal to control coupling of the electricmachine to the at least one wheel of the axle; and processing meansarranged to determine a coupling state of the electric machine to the atleast one wheel of the axle in dependence on the at least one attributesignal, wherein the processing means is arranged to control the outputmeans to output the coupling signal indicative of the determinedcoupling state, wherein the processing means is arranged to defercontrolling the output means to output the coupling signal indicative ofa change in the coupling state until expiry of a predetermined period oftime since a last change in the coupling state indicated by the couplingsignal. Advantageously excessively frequent changes in coupling stateare prevented.

The predetermined period of time may be 30 seconds. Advantageouslyfrequent changes in coupling state are prevented.

The predetermined period of time may be 10 seconds. The predeterminedperiod of time may be 5 seconds. Advantageously regular changes incoupling state are prevented.

The change in the coupling state may be from coupled to decoupled. Thechange in the coupling state may be from decoupled to coupled.Advantageously the changes in coupling state from decoupled to coupledor coupled to decoupled are prevented.

The at least one attribute signal may comprise a speed signal indicativeof a speed of the vehicle. Advantageously the coupling state may bedetermined in dependence on a speed of the vehicle.

The at least one attribute signal may comprise a driving mode signalindicative of a driving mode of the vehicle. Advantageously the couplingstate may be determined in dependence on a driving mode of the vehicle.

According to an aspect of the invention, there is provided a powertraincomprising the system as described above.

According to an aspect of the invention, there is provided a vehiclecomprising the control system as described above or the powertrain asdescribed above.

The v electric machine may be arranged to be selectively coupleable toprovide torque to at least one wheel of a first axle of the vehicle. Thevehicle comprises a second motive power source arranged to providetorque to at least one wheel of a second axle of the vehicle. The secondmotive power source may comprise a second electric machine.

According to an aspect of the invention, there is provided a method ofcontrolling coupling of an electric machine to provide torque to atleast one wheel of an axle of a vehicle, the method comprising:receiving at least one attribute signal indicative of one or moreattributes of the vehicle; determining a coupling state of the electricmachine to the at least one wheel of the axle in dependence on the atleast one attribute signal; and outputting a coupling signal indicativeof the determined coupling state to control coupling of the electricmachine to the at least one wheel of the axle, wherein the outputtingcomprises deferring outputting the coupling signal indicative of achange in the coupling state until expiry of a predetermined period oftime since a last change in the coupling state indicated by the couplingsignal.

The predetermined period of time may be 30 seconds. The predeterminedperiod of time may be 10 seconds. The change in the coupling state maybe from coupled to decoupled. The change in the coupling state may befrom decoupled to coupled.

The at least one attribute signal may comprise a speed signal indicativeof a speed of the vehicle. The at least one attribute signal maycomprises a driving mode signal indicative of a driving mode of thevehicle.

According to another aspect of the invention, there is provided computersoftware which, when executed by a computer, is arranged to perform amethod as described above. The computer software may be stored on acomputer-readable medium. The computer software may be tangibly storedon the computer readable medium.

Within the scope of this application it is expressly intended that thevarious aspects, embodiments, examples and alternatives set out in thepreceding paragraphs, in the claims and/or in the following descriptionand drawings, and in particular the individual features thereof, may betaken independently or in any combination. That is, all embodimentsand/or features of any embodiment can be combined in any way and/orcombination, unless such features are incompatible. The applicantreserves the right to change any originally filed claim or file any newclaim accordingly, including the right to amend any originally filedclaim to depend from and/or incorporate any feature of any other claimalthough not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 shows a vehicle according to an embodiment of the invention;

FIG. 2 shows a system according to an embodiment of the invention;

FIG. 3 shows a control system according to an embodiment of theinvention;

FIG. 4 shows an illustration of modules of the control system accordingto embodiments of the invention;

FIG. 5 shows a method according to an embodiment of the invention;

FIG. 6 illustrates operation of a module according to an embodiment ofthe invention;

FIG. 7 further illustrates operation of a module according to anembodiment of the invention;

FIG. 8 illustrates operation of another module according to anembodiment of the invention;

FIG. 9 shows a method according to another embodiment of the invention;

FIG. 10 shows a method according to still another embodiment of theinvention;

FIG. 11 shows a method according to yet another embodiment of theinvention;

FIG. 12 shows a method according to further embodiment of the invention;

FIG. 13 shows a method according to a yet further embodiment of theinvention;

FIG. 14 shows a method according to still further embodiment of theinvention;

FIG. 15 illustrates operation of a module according to an embodiment ofthe invention; and

FIG. 16 illustrates operation of the system according to an embodimentof the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a vehicle 100 according to an embodiment of theinvention. The vehicle 100 provides space within a cabin of the vehicle100 for one or more occupants. In some embodiments, the vehicle 100 maybe manually driven by one of the occupants representing a driver of thevehicle 100, although the vehicle 100 may have an at least partlyautonomous driving capability in some embodiments. The vehicle 100 is anat least partly electric-powered vehicle 100, as will be explained, withan internal combustion engine and one or more electric machines ortraction electric motors for providing motive torque, thereby thevehicle being a hybrid electric vehicle (HEV). In some embodiments thevehicle 100 may be entirely electric powered i.e. a battery electricvehicle (BEV) without an internal combustion engine.

FIG. 2 illustrates a system 20 for a parallel HEV 10. The system 20defines, at least in part, a powertrain of the HEV. The system 20comprises a control system 208. The control system 208 comprises one ormore controllers. The control system 208 may comprise one or more of: ahybrid powertrain control module; an engine control unit; a transmissioncontrol unit; a traction battery management system; and/or the like.

The system 20 comprises an engine 202. The engine 202 is a combustionengine. The illustrated engine 202 is an internal combustion engine. Theillustrated engine 202 comprises three combustion chambers, however adifferent number of combustion chambers may be provided in otherexamples.

The engine 202 is operably coupled to the control system 208 to enablethe control system 208 to control output torque of the engine 202. Theoutput torque of the engine 202 may be controlled by controlling one ormore of: air-fuel ratio; spark timing; poppet valve lift; poppet valvetiming; throttle opening position; fuel pressure; turbocharger boostpressure; and/or the like, depending on the type of engine 202.

The system 20 comprises a vehicle transmission arrangement 204 forreceiving output torque from the engine 202. The vehicle transmissionarrangement 204 may comprise an automatic vehicle transmission or asemi-automatic vehicle transmission. The vehicle transmissionarrangement 204 comprises a fluid-coupling torque converter 217 betweenthe engine 202 and a gear train.

The system 20 may comprise a differential (not shown) for receivingoutput torque from the gear train. The differential may be integratedinto the vehicle transmission arrangement 204 as a transaxle, orprovided separately.

The engine 202 is mechanically connected or connectable to a first setof vehicle wheels (FL, FR) via a first torque path 220. The first torquepath 220 extends from an output of the engine 202 to the vehicletransmission arrangement 204, then to axles/driveshafts, and then to thefirst set of vehicle wheels (FL, FR). In a vehicle overrun and/orfriction braking situation, torque may flow from the first set ofvehicle wheels (FL, FR) to the engine 202. Torque flow towards the firstset of vehicle wheels (FL, FR) is positive torque, and torque flow fromthe first set of vehicle wheels (FL, FR) is negative torque.

The illustrated first set of vehicle wheels (FL, FR) comprises frontwheels, and the axles are front transverse axles. Therefore, the system20 is configured for front wheel drive by the engine 202. In anotherexample, the first set of vehicle wheels (FL, FR) comprises rear wheels(RL, RR). The illustrated first set of vehicle wheels (FL, FR) is a pairof vehicle wheels, however a different number of vehicle wheels could beprovided in other examples.

In the illustrated system 20, no longitudinal (centre) driveshaft isprovided, to make room for hybrid vehicle components. Therefore, theengine 202 is not connectable to a second set of rear wheels (rearwheels RL, RR in the illustration). The engine 202 may be transversemounted to save space.

A torque path connector 218 such as a clutch is provided inside and/oroutside a bell housing of the vehicle transmission arrangement 204. Theclutch 218 is configured to connect and configured to disconnect thetorque path 220 between the engine 202 and the first set of vehiclewheels (FL, FR). The system 20 may be configured to automaticallyactuate the clutch 218 without user intervention.

The system 20 comprises a first electric traction motor 216. The firstelectric traction motor 216 may be an alternating current inductionmotor or a permanent magnet motor, or another type of motor. The firstelectric traction motor 216 is located to the engine side of the clutch218.

The first electric traction motor 216 may be mechanically coupled to theengine 202 via a belt or chain. For example, the first electric tractionmotor 216 may be a belt integrated starter generator (BiSG). In theillustration, the first electric traction motor 216 is located at anaccessory drive end of the engine 202, opposite a vehicle transmissionend of the engine 202. In an alternative example, the first electrictraction motor 216 is a crankshaft integrated motor generator, locatedat a vehicle transmission end of the engine 202.

The first electric traction motor 216 is configured to apply positivetorque and configured to apply negative torque to a crankshaft of theengine 202, for example to provide functions such as: boosting outputtorque of the engine 202; deactivating (shutting off) the engine 202while at a stop or coasting; activating (starting) the engine 202; andregenerative braking in a regeneration mode. In a hybrid electricvehicle mode, the engine 202 and first electric traction motor 216 areboth operable to supply positive torque simultaneously to boost outputtorque. The first electric traction motor 216 may be incapable ofsustained electric-only driving, although in other embodiments the firstelectric traction motor 216 may be capable of electric only drivingparticularly an embodiment without the engine 202. One or both of theengine 202 and the first electric traction motor 216 are able to providetorque to a first axle 221 of the vehicle.

However, when the torque path 220 between the engine 202 and the firstset of vehicle wheels (FL, FR) is disconnected, a torque path 220between the first electric traction motor 216 and the first set ofvehicle wheels (FL, FR) is also disconnected.

FIG. 2 illustrates a second electric traction motor 212 configured toenable at least an electric vehicle mode comprising electric-onlydriving. In some, but not necessarily all examples, a nominal maximumtorque of the second electric traction motor 212 is greater than anominal maximum torque of the first electric traction motor 216.

Even if the torque path 220 between the engine 202 and the first set ofvehicle wheels (FL, FR) is disconnected by the clutch 218, the vehicle10 can be driven in electric vehicle mode because the second electrictraction motor 212 is connected to at least one vehicle wheel. The atleast one vehicle wheel may be one, or both, of the rear wheels (RL, RR)of the vehicle 100 associated with a second axle 222 of the vehicle 100.

The illustrated second electric traction motor 212 is configured toprovide torque to the illustrated second set of vehicle wheels (RL, RR)of the second axle 222 of the vehicle. The second set of vehicle wheels(RL, RR) comprises vehicle wheels not from the first set of vehiclewheels (FL, FR). The illustrated second set of vehicle wheels (RL, RR)comprises rear wheels, and the second electric traction motor 212 isoperable to provide torque to the rear wheels (RL, RR) via reartransverse axles forming the second axle 222. Therefore, the vehicle 10may be rear wheel driven in electric vehicle mode.

The control system 208 may be configured to disconnect the torque path220 between the engine 202 and the first set of vehicle wheels (FL, FR)in electric vehicle mode, to reduce parasitic pumping energy losses. Forexample, the clutch 218 may be opened. In the example of FIG. 2 , thismeans that the first electric traction motor 216 will also bedisconnected from the first set of vehicle wheels (FL, FR).

Another benefit of the second electric traction motor 212 is that thesecond electric traction motor 212 may also be configured to operable ina hybrid electric vehicle mode, to enable four-wheel drive operationdespite the absence of a centre driveshaft.

The second electric traction motor 212 may be selectively coupled to oneor both wheels RL, RR of the second axle 222. Coupling of a torque pathbetween the second electric traction motor 212 and the one or bothwheels RL, RR of the second axle 222 may be achieved via a second clutch219. The second clutch 219 may be controlled to open, such as via anactuator under control of a received signal, to disconnect the torquepath between the second electric traction motor 212 and the one or bothwheels (RL, RR) of the second axle 222. In some embodiments the secondclutch 219 may be a dog cutch.

Thus it will be appreciated that the second electric traction motor 212is arranged to be selectively coupleable to provide torque to at leastone wheel (RL, RR) of an axle of the vehicle 100. In some embodiments,the vehicle 100 comprises another motive power source arranged toprovide torque to at least one wheel (FL, FR) of another axle of thevehicle 100. In the illustrated embodiment the another motive powersource power source comprises another electric machine 216 in the formof the first electric traction motor 216. The another motive powersource may, in some embodiments, comprise an internal combustion engine202 which may provide positive torque alone or in combination with thefirst electric traction motor 216.

In order to store electrical power for the electric traction motors 212,216, the system 20 comprises a traction battery 200. The tractionbattery 200 provides a nominal voltage required by electrical powerusers such as the electric traction motors. If the electric tractionmotors 212, 216 run at different voltages, DC-DC converters (not shown)or the like may be provided to convert voltages.

The traction battery 200 may be a high voltage (HV) battery. Highvoltage traction batteries provide nominal voltages in the hundreds ofvolts, as opposed to traction batteries for mild HEVs which providenominal voltages in the tens of volts. The traction battery 200 may havea voltage and capacity to support electric only driving for sustaineddistances. The traction battery 200 may have a capacity of severalkilowatt-hours, to maximise range. The capacity may be in the tens ofkilowatt-hours, or in the hundreds of kilowatt-hours.

Although the traction battery 200 is illustrated as one entity, thefunction of the traction battery 200 could be implemented using aplurality of small traction batteries in different locations on thevehicle 10.

In some examples, the first electric traction motor 216 and secondelectric traction motor 212 may be configured to receive electricalenergy from the same traction battery 200. By pairing the first (mild)electric traction motor 216 to a high-capacity battery (tens to hundredsof kilowatt-hours), the first electric traction motor 216 may be able toprovide the functionality of the methods described herein for sustainedperiods of time, rather than for short bursts. In another example, theelectric traction motors 212, 216 may be paired to different tractionbatteries.

Finally, the illustrated system 20 comprises one or more inverters. Twoinverters 210, 214 are shown, one for each electric traction motor 212,216. In other examples, one inverter or more than two inverters could beprovided.

It can be appreciated from the foregoing that the vehicle 100 may beprovided with motive torque from a combination of sources. Embodimentsof the present invention relate to determining which of the sources ofmotive torque to utilise.

FIG. 3 illustrates a control system 300 according to an embodiment ofthe invention. The control system 300 may be formed by one or morecontrollers 305. The control system 300 illustrated in FIG. 2 comprisesone electronic controller 305 although it will be appreciated that thisis merely illustrative. The, or each, controller 305, comprises aprocessing means 310 and a memory means 320. The processing means 310may be one or more electronic processors 310 or processing devices 310,such as CPUs, for executing computer readable instructions. The memorymeans 320 may be one or more memory devices 320. The one or more memorydevices 320 may store computer-readable instructions for execution bythe at least one processing device 310.

The controller 305 comprises an input means 330 and an output means 340.The input means 330 is arranged to receive one or more signals 335. Theinput means 330 may be an electrical input to the controller 305 forreceiving one or more electrical signals 335. The output means 340 isarranged to output at least one signal 345, which is provided in FIG. 3to one or both of the second clutch 219 and second electric tractionmotor 212 to control coupling to the second torque path to providetorque to one or both wheels of the second axle 222. The output means340 is an electrical output of the controller 305. The output means 340is operable by the processing device 310 to output the signal 345 undercontrol thereof. The signal 345 may cause the second electric tractionmotor 212 to ‘spin-up’ or accelerate to a rotation speed suitable tocouple with the second axle 222 i.e. bearing in mind that the vehicle100 may be in motion through torque provided by the first electrictraction motor 216 and/or engine 202. The signal 345 may cause closingof the second clutch 219 to couple the second electric traction motor212 to the second torque path.

The electrical input 330 and output 340 of the controller 305 may beprovided to/from a communication bus or network of the vehicle, such asa CANBus or other communication network which may, for example, beimplemented by an Internet Protocol (IP) based network such as Ethernet,or FlexRay or a Single Edge Nibble Transmission (SENT) protocol,although other protocols may be used.

FIG. 4 schematically illustrates a portion of the controller 305comprising the input means 330 and output means 340 of the system 300.FIG. 4 illustrates inputs 410, 420, 430, 440, 450, 460, 470 to the inputmeans 330 of the controller 305 which form the signal 335 illustrated inFIG. 3 . FIG. 4 further illustrates modules 510, 520, 530, 540, 550,560, 570, or functional units, which may operatively execute on theprocessing device 310 of the controller 305. Each of the inputs 410,420, 430, 440, 450, 460, 470 provides information relating to one ormore aspects or attributes of the vehicle 100 or the powertrain 20thereof.

The inputs 410, 420, 430, 440, 450, 460, 470 may comprise one more ofone or more speed signals 410, a temperature signal 420, a fault-derivedcoupling state request (FDCSR) signal 430, a driving mode (DM) signal440, a state of charge (SoC) signal 450 and an inhibit signal 460 whichprovide information or data on which a desired coupling state isdetermined by one or more of the modules 510, 520, 530, 540, 550, 560,570 as will be explained. The desired coupling state is a desiredcoupling of the torque path between the second electric traction motor212 and the one or both wheels RL, RR of the second axle 222 of thevehicle 100 which is determined by one or more of the modules 510, 520,530, 540, 550, 560, 570.

The one or more speed signals 410 is indicative of one or more of aspeed of the vehicle 100 i.e. a speed of the vehicle 100 over ground, awheel speed signal indicative of a speed of rotation of one or morewheels of the vehicle and a motor speed signal indicative of a speed ofone or both of the speed of the first and second electric tractionmotors 216, 212.

The temperature signal 420 is indicative of one or more of an ambienttemperature and a temperature of one or more units, or a temperature offluids associated with one or more units, particularly fluids used forcooling said units i.e. coolant fluid, of the vehicle 100. For examplethe coolant fluid may be a coolant fluid of one or both tractionelectric motors 212, 216. In some embodiments, the temperature signal420 comprises a temperature associated with one more units of thepowertrain. In some embodiments, the temperature associated with onemore units of the powertrain comprises a temperature of one or more ofone or both of the inverters 210, 214, one or both of the electrictraction motors 212, 216, a coolant temperature, and an indication of atemperature of the traction battery 200. The indication of thetemperature of the traction battery 200 may be indicative of a powercapability of the traction battery 200, which is a function oftemperature and a State of Charge (SoC) of the traction battery 200.Thus in some embodiments the temperature signal 420 may comprise asignal indicative of the power capability of the traction battery 200,this being indicative of temperature.

The fault-derived coupling state request signal (FDCSR) 430 isindicative of a request for a coupling state derived in determination ofa fault associated with the vehicle 100, such as a fault associated withthe powertrain. For example, where a fault associated with the secondclutch 219 is detected by a fault management module (not shown), thefault management module may request that a coupling state of coupled ordecoupled in order to control a state of the clutch 219 i.e. open orclosed, in order to manage or resolve the fault. Other faults may beappreciated to cause a desired coupling state to manage or amelioratethe fault. In some embodiments, a fault management module 530 may beexecuted upon the processing device 310 and thus the FDSCR signal 430may be generated internal to the controller 305.

The driving mode signal 440 may be indicative of a driving mode of thevehicle 100 which may be automatically determined, such as by anintelligent driving mode or terrain response (TR) determination unit, anautonomous driving controller, such as an ADAS system, or selected by anoccupant of the vehicle 100. The driving mode signal 440 may beindicative of selection of an efficiency-based driving mode i.e. toprovide minimal fuel and/or energy usage, a four wheel-drive drivingmode, such as where a number of driven wheels may be automaticallyselected, and a selected driving gear i.e. neutral, drive (D), reverse(R) etc.

The state of charge (SoC) signal 450 is indicative of the SoC of thetraction battery 200.

The inhibit signal 460 is indicative of one or more inhibited couplingstates. For example, the inhibit signal 460 may indicate that a state ofcoupled is inhibited to prevent coupling of the second electric tractionmotor 212 to the one or both wheels (RL, RR) of the second axle 222, orthat a state of decoupled is inhibited to prevent decoupling of thesecond electric traction motor 212 from the one or both wheels (RL, RR)of the second axle 222.

The inputs 410, 420, 430, 440, 450, 460, 470 may, in some embodiments,comprise a coupling status signal 470 which is indicative of an actualcoupling status of the second electric traction motor 212 to the one orboth wheels of the second axle 222. In some embodiments, the couplingstatus signal 470 has states of coupled and decoupled indicative therespective coupling. The coupling status signal 470 reports the physicalstatus of the coupling of the second electric traction motor 212 to thesecond torque path via the second axle 222 and is thus indicative ofsuccessful coupling or decoupling of the second electric traction motor212.

In some embodiments, the modules 510, 520, 530, 540, 550, 560, 570comprise a high-speed module 510, a low-speed module 520, a faultmanagement module (FMM) 530, an anti-fussiness module 540, an inhibitmodule 550, a driving mode module (DMM) 560 and an arbitrator 570. Itwill be appreciated that not all modules are present in all embodiments,thus embodiments of the present invention may comprise one or more ofthe aforementioned modules. Each of the modules will be explained below.Each of the high-speed module 510, the low-speed module 520, the faultmanagement module 530, the anti-fussiness module 540, the inhibit module550, the efficiency module 560, as present in the relevant embodiment,may determine a respective desired coupling state. An indication of thedesired coupling state is provided to the arbitrator 570 to determinethe coupling state of the electric machine 212 to the axle 222 i.e. asan arbitrated coupling state.

An embodiment of the high-speed module (HSM) 510 will now be explainedwith reference to FIGS. 5 & 6 . The HSM 510 is operatively executable bythe processing device 310 to determine a coupling state of the electricmachine 212 to the at least one wheel of the axle 222 in dependence onthe speed signal 410 indicative of the speed of the vehicle 100. In someembodiments, the HSM 510 and the arbitrator 570 are arranged to causethe controller 305 to output a coupling signal 345 to control couplingof the second electric traction motor 212 to the at least one wheel ofthe axle 222 dependent on the speed signal 410 as will be explained. TheHSM 510 is arranged to cause decoupling of the second electric tractionmotor 212 from the at least one wheel of the axle 222 a high-speeds ofthe vehicle 100 which, advantageously, prevents rotation of the secondelectric traction motor 212 at excessive speeds which may damage thesecond electric traction motor 212.

FIG. 5 illustrates a method 600 according to an embodiment of theinvention which may be performed by the HSM 510 executed by theprocessing device 310 of the controller 305. The method 600 will beexplained with reference to FIG. 6 which illustrates a speed of thevehicle 100, as indicated by the speed signal 410, over a period oftime. Also illustrated in a lower portion of FIG. 6 is a desiredcoupling signal 515 output by the HSM 510 which represents a request730, 740 for the desired coupling state from the HSM 510 determined independence on the speed signal 410.

The method 600 comprises a step 610 of receiving one or more signals,such as data representing the one or more signals, at the HSM 510. Inthe illustrated embodiment the HSM 510 is arranged to receive the speedsignal 410, which as discussed above may be indicative of the speed ofthe vehicle 100. In some embodiments, the HSM 510 is arranged to receivethe temperature signal 420 as discussed above. In some embodiments, theHSM 510 is arranged to receive the SoC signal 460 indicative of thestate of charge of one or more traction batteries 200 for providingelectrical power to the traction electric machines 212, 216. In someembodiments, the HSM 510 may receive a signal indicative of a powerlimit or capability of the traction battery 200 which, as discussedabove, is indicative of the temperature of the traction battery 200.

Step 620 comprises determining a desired coupling state of the secondelectric traction motor 212 to the at least one wheel (RL, RR) of thesecond axle 222 in dependence on the speed signal 410. Step 620comprises determining whether the speed of the vehicle 100 is equal toor greater than a first high-speed threshold 710 shown in FIG. 6 . Thusstep 620 comprises comparing the speed of the vehicle 100 against one ormore thresholds 710, 720, where the one or more thresholds 710, 720comprise the first high-speed threshold 710. In some embodiments, theone or more thresholds 710, 720 comprise a second high-speed threshold720. The second high-speed threshold 720 represents a vehicle speedlower than the first high-speed threshold 710. The first 710 and second720 high-speed thresholds are illustrated in FIG. 6 .

If the speed of the vehicle 100 is equal to or greater than a firsthigh-speed threshold 710 then the method 600 moves to step 630. If,however, the speed of the vehicle 100 is less than the first high-speedthreshold 710 then the method 600 moves to step 640.

In the example of FIG. 6 , the method 600 progresses to step 640 priorto time t₁. Prior to time t₁ as will be appreciated the vehicle 100 isgenerally accelerating which may be caused by positive torque applied bythe first electric traction motor 216 and/or engine 202, and the secondelectric traction motor 212 which is coupled to the second torque pathvia the second axle 222.

In step 630 the desired coupling state is determined as decoupled independence on the speed signal 410 being indicative of a vehicle speedequal to or greater than the first high-speed threshold 710. In step 630the HSM 510 may output an indication 515 of the desired coupling stateof decoupled to the arbitrator 570 indicative of a request to decouple740 the second electric traction motor 212 from the second axle 222. Theindication 515 of the desired coupling state of decoupled 740 may bereferred to as the high-speed coupling state request 515, 740. Thearbitrator 570 may in some embodiments arbitrate between multiplerequests for desired coupling states as will be explained. In theabsence of any other competing requests from other modules, thearbitrator 570 is arranged to output, via the output means 340, thehigh-speed coupling state request 515 for the decoupled state 740 asoutput signal 345. In some embodiments, the high-speed coupling staterequest 515 may be provided from the HSM 510 directly to the outputmeans 340 of the controller 305.

After time t₁, i.e. once the speed of the vehicle 100 exceeds the firsthigh-speed threshold 710, it has been determined that it is desirable todecouple the second electric traction motor 212. Continued coupling ofthe second electric traction motor 212 to the wheel(s) of the vehicle100 causes the second electric traction motor 212 to exceed apredetermined rotation speed. The predetermined rotation speed may be amotor speed of 12,000 rpm, although it will be appreciated that otherpredetermined rotation speeds may be selected. The predeterminedrotation speed may correspond to a vehicle speed of 140 kmh⁻¹ althoughit will be appreciated that this depends on a gearing between the secondelectric traction motor 212 and the wheels of the vehicle 100 and adiameter of the wheels. Furthermore, in some embodiments, the vehiclespeed corresponding to the first high speed threshold 710, and thus therotation speed of the second electric traction motor 212, may bedetermined in dependence on temperature as will be explained withreference to FIG. 7 .

The output means 340 of the controller 305 is arranged to output thecoupling signal 345, 730, 740 indicative of a request to decouple 740the second electric traction motor 212 from the at least one wheel ofthe second axle 222 in dependence on the desired coupling state beingdecoupled.

If, in step 620, the speed of the vehicle 100 is less than the firsthigh speed threshold 710, the method moves to step 640. In step 640 itis determined whether the speed of the vehicle 100 is less than or equalto the second high speed threshold 720. If the speed of the vehicle 100is less than or equal to the second high speed threshold 720 the methodmoves to step 660.

In step 660 the HSM 510 is arranged not to request a desired couplingstate of the second electric machine 212. The HSM 510 outputs a requestfor a coupling state to the arbitrator 570 or may, as illustrated inFIG. 5 , output a ‘no-request’ signal 730 to the arbitrator 570, wherethe no-request signal 730 is indictive of the HSM 510 not requesting aspecific coupling state of the second electric traction motor 212 to theone or more wheels of the second axle 222. Thus, prior to time t₁ inFIG. 6 , the HSM 510 outputs the no-request signal 730 to the arbitrator570, or may output no signal to the arbitrator 570 in other embodiments.The arbitrator 570 may have a default coupling state. The defaultcoupling state may be coupled i.e. for the second electric tractionmotor 212 to be coupled to the torque path of the second axle 222. Thuswhen either a ‘no-request’ signal 730, or no request signal is receivedby the arbitrator 570, the arbitrator 570 may output a determinedcoupling request via the output means 340.

In some embodiments, the HSM 510 is arranged to output the couplingsignal 345, indicative of a request to couple the second electrictraction motor 212 machine to the at least one wheel of the second axle222. It will be appreciated that the HSM 510 may, in some embodiments,request the default state of coupled when the speed signal 410 isindicative of a low vehicle speed.

In some embodiments, the HSM 510 may apply hysteresis to the speedsignal 410 to determine the coupling state. That is, the coupling stateof decoupled may be determined for a vehicle speed greater than that atwhich the second electric traction motor 212 is recoupled to the torquepath via the second axle 222 i.e. above the second high-speed threshold720. Advantageously this assists in preventing ‘hunting’ or ‘flickering’between the decoupled and coupled states as the speed of the vehiclevaries around (above and below) the first high speed threshold 710. Useof the second high speed threshold 720 provides the hysteresis in someembodiments. As can be appreciated from FIG. 6 , between t₁ and prior totime t₂ the vehicle deaccelerates from a peak speed, such that the speedsignal 410 drops below the first high speed threshold 710. As can beappreciated from the lower portion of FIG. 6 , the ‘no-request’ signal730 is not output immediately upon the speed of the vehicle 100 fallingbelow the first high-speed threshold 710.

Instead, in a region between the first and second high speed thresholds710, 720 the coupling state of decoupled 740 is maintained until thevehicle speed falls below the second high-speed threshold 720. In step650, which is reached when the vehicle speed is between the first andsecond high speed thresholds 710, 720 the desired coupling state isdetermined in dependence on the speed signal 410 in dependence on a lastintersected of the first and second high-speed thresholds 710, 720.Thus, prior to time t₂ when the speed signal 410 is below the firsthigh-speed threshold 710 the coupling state is determined in step 650 asdecoupled based on last-intersecting the first high speed threshold 710.Thus the method moves to step 630. Similarly, prior to time t₁, when thespeed signal 410 is above the second high-speed threshold 720, themethod moves to step 660 wherein the ‘no request’ output signal 730 ismaintained such that the arbitrator 570 in the example embodimentdetermines the coupling state as coupled.

Thus it can be appreciated that embodiments of the invention selectcoupling of the second electric traction motor 212 in dependence on thespeed of the vehicle 100.

FIG. 7 illustrates motor speed, i.e. speed (RPM) of the second electrictraction motor 121, against temperature according to an embodiment ofthe invention. Illustrated in FIG. 7 is the first high speed threshold710 which, according to some embodiments of the invention variesdependent upon temperature. As described above, in some embodiments ofthe invention the controller 305 receives the temperature signal 420. Insome embodiments, the first high speed threshold 710 adopts a firstvalue 710 between first and second temperatures 740, 750. The firsttemperature 740, below which one or both of the first and secondhigh-speed thresholds 710, 720 reduces may correspond to acold-temperature, such as a temperature below 0° C., such as −5° C.,although it will be realised that other temperatures may be selected. Itwill be appreciated that, although not illustrated, the second highspeed threshold 720 may follow the first high speed threshold 710.

Below the first temperature 740, in some embodiments the first highspeed threshold 710 reduces i.e. to value 810, such that the couplingstate of the second electric traction motor 212 is determined asdecoupled at a lower speed, as illustrated. In some embodiments, one orboth of the first and second high-speed thresholds 710, 720 may reduceproportional to temperature during one or more temperature regions.Advantageously, the reduction in the first high speed threshold 710, 810allows for changes in, for example, coolant of the second electrictraction motor 212 or a reduced viscosity of fluids associated with thesecond torque path via the second axle 222, such that rotation of themotor 212 may consume more energy and therefore lower-speed decouplingis more efficient. In the embodiment shown in FIG. 7 , the first highspeed threshold 710, 810 is arranged to decrease in dependence ontemperature over a first temperature range 740, 730. The temperaturerange may be between −10° C. and −20° C., although other temperatureranges may be selected. In other embodiments, the first high speedthreshold 710 may reduce instantaneously, however advantageously havinga gradual change may be less noticeable to occupants of the vehicle 100.Below a third temperature 730 the first thigh speed threshold 810corresponds to a minimum threshold speed 810.

Similarly, in some embodiments, above the second temperature 750 thefirst thigh speed threshold 710 is arranged to decrease in dependence ontemperature over a second temperature range 750, 760 to a fourthtemperature 760. Above the fourth temperature 760 the first thigh speedthreshold 710 adopts a constant value 820 in some embodiments, which maybe different to the minimum threshold speed 810 as shown in FIG. 7 ,although in other embodiments the two speeds 810, 820 may be equal.Advantageously the reduction in the first thigh speed threshold 710, 820at higher speeds may reduce cooling issues associated with the secondelectric traction motor 212. The temperature 750 may be at least 25° C.or at least 35° C., such as in some embodiments a temperature of between50° C. and 60° C.

As described above, in some embodiments, the controller 305 is arrangedto receive the SoC signal 450. In some embodiments, one or both thefirst high speed threshold 710 and second high speed threshold 720 isdetermined in dependence on the SoC of the traction battery 200. Asdescribed above, in some embodiments, the arbitrator 570 may be arrangedto achieve the default coupling state of coupled in absence of a requestfrom the HSM 510 for the decoupled state. In this way, the HSM 510 andarbitrator 570 operate to decouple the second electric traction motor212 when the vehicle 100 speed is above first high speed threshold 710and coupled when the vehicle 100 speed is below the second high speedthreshold 720. To couple the second electric traction motor 212 to thesecond axle in some embodiments the second electric traction motor 212is required to ‘spin-up’ or accelerate from a low, such as zero,rotation speed to generally a speed of rotation of the rear axle 222before the second clutch 219 can be closed to couple the second electrictraction motor 212 to the axle 222. As can be appreciated, acceleratingthe second electric traction motor 212 consumes energy from the tractionbattery 200. When the vehicle 100 is operative with a traction battery200 having a low SoC, one or both the first high speed threshold 710 andsecond high speed threshold 720 may be reduced in dependence on the SoC.Advantageously, by reducing the speed corresponding to one or both thefirst high speed threshold 710 and second high speed threshold 720, thesecond electric traction motor 212 is only required to ‘spin-up’ to alower rotation speed to recouple to the second axle 222, therebyrequiring less energy consumption when the traction battery 200 has alower SoC.

An embodiment of the low-speed module (LSM) 520 will now be explainedwith reference to FIGS. 8 and 9 . The LSM 520 is operatively executableby the processing device 310 to determine a coupling state of theelectric machine 212 to the at least one wheel of the second axle 222 independence on the speed signal 410 indicative of the speed of thevehicle 100. In some embodiments, the LSM 510 and arbitrator 570 arearranged to cause the controller 305 to output a coupling signal 345 tocontrol coupling of the electric machine 212 to the at least one wheelof the second axle 222 dependent on the speed signal 410, as will beexplained. As will be explained the LSM 520 is arranged to causecoupling of the electric machine 212 to the at least one wheel of theaxle 222 a low-speeds which, advantageously, enables the electricmachine 212 to provide motive torque for the vehicle at low speeds,especially from stationary. Furthermore, the LSM 520 is arranged tocontrol the coupling of the electric machine to avoid, or reduce,undesirable characteristics which may be noticeable to an occupant ofthe vehicle 100 as will be explained.

FIG. 9 illustrates a method 1000 according to an embodiment of theinvention which may be performed by the LSM 520 executed by theprocessing device 310 of the controller 305. The method 1000 will beexplained with reference to FIG. 8 which illustrates a speed of thevehicle 100, as indicated by the speed signal 410, over a period oftime. Also illustrated in a lower portion of FIG. 8 is a desiredcoupling signal 525 output by the LSM 520 which represents a request730, 750 for the desired coupling state from the LSM 520 determined independence on the speed signal 410.

The method 1000 comprises a step 1010 of receiving one or more signals,such as data representing the one or more signals, at the LSM 520. Inthe illustrated embodiment the LSM 520 is arranged to receive the speedsignal 410 indicative of the speed of the vehicle 100.

Step 1020 comprises determining a desired coupling state of the secondelectric traction motor 212 to the at least one wheel (RL, RR) of thesecond axle 222 in dependence on the speed signal 410. Step 1020comprises determining whether the speed of the vehicle 100 is equal toor less than a first low-speed threshold (LST) 910. Thus step 1020comprises comparing the speed of the vehicle 100 against one or morethresholds 910, 920, where the one or more thresholds 910, 920 comprisethe first LST 910. In some embodiments, the one or more low-speedthresholds 910, 920 comprise a second LST 920, as shown in FIG. 8 . Thesecond LST 920 represents a vehicle speed greater than the first LST910. The first 910 and second 920 LSTs are illustrated in FIG. 8 .

In step 1030 the desired coupling state is determined as coupled independence on the speed signal 410 being indicative of a vehicle speedequal to or below than the first LST 910. In step 1030 the LSM 520 mayoutput an indication 525 of the desired coupling state of coupled to thearbitrator 570 indicative of a request to couple 750 the second electrictraction motor 212 to the second axle 222. The indication 525 of thedesired coupling state of coupled 750 may be referred to as thelow-speed coupling state request 525, 750. The arbitrator 570 may insome embodiments arbitrate between multiple requests for desiredcoupling states. In the absence of any other competing requests fromother modules, the arbitrator 570 is arranged to output, via the outputmeans 340, the low-speed coupling state request 525 for the coupledstate 750 as output signal 345. In some embodiments, the low-speedcoupling state request 525, 750 may be provided from the LSM 520directly to the output means 340 of the controller 305.

Referring to FIG. 8 , after time t₃, i.e. once the speed of the vehicle100 is equal to or below the first LST 910, it has been determined thatit is desirable to couple the second electric traction motor 212. Forexample, it can be envisaged that the vehicle 100 is about to stop andthat torque from the second electric traction motor 212 will be usefule.g. for a standing start. A predetermined vehicle speed correspondingto the first LST 910 may be a vehicle speed of 10 kmh⁻¹ although it willbe appreciated that other vehicle speeds may be selected. In someembodiments, the vehicle speed corresponding to the first LST 910 may beselected or determined based on a deacceleration rate of the vehicle100, which may be determined based on a rate of change of the speedsignal 410. In the presence of a large deceleration i.e. above adeacceleration threshold the vehicle speed corresponding to the firstLST 910 may be increased to advantageously allow for coupling of thesecond electric traction motor 212 prior to the vehicle 100 stopping.

The output means 340 of the controller 305 is arranged to output thecoupling signal 345, 750 indicative of a request to couple 750 thesecond electric traction motor 212 to the at least one wheel of thesecond axle 222 in dependence on the desired coupling state beingcoupled, as in step 1030.

In some instances, due to a default state being coupled as shown inTable 1 below, the request to couple 750 shown in FIG. 8 output as aresult of the speed of the vehicle dropping through the LST 910 willhave no practical effect (change in state) as the second electrictraction motor 212 will already be coupled to the second axle 222 as aresult of the default state being coupled. However, in some instances,the second electric traction motor 212 will be decoupled from the secondaxle 222 when the speed of the vehicle drops through the LST 910. Insuch situations, the arbitrator 570 may determine an arbitrated couplingstate with respect to the LTS 910 in dependence on a reason why thesecond electric traction motor 212 is disconnected. If the arbitratedcoupling state is decoupled whilst the vehicle speed is above the LST910 for a high priority reason, such as a fault, then the arbitrator 570will not change the arbitrated coupling state to coupled responsive tothe request to couple 750 from the LSM 520. However, if the reason forthe decoupled state is lower priority, such as a preferential reason,the arbitrator 570 may change the arbitrated coupling state to coupledresponsive to the request to couple 750 from the LSM 520.

If, in step 1020, if the speed of the vehicle 100 is greater than thefirst LST 910, the method moves to step 1040. In step 1040 it isdetermined whether the speed of the vehicle 100 is greater than or equalto the second LST 920. If the speed of the vehicle is greater than orequal to the second LST 920 the method moves to step 1060.

In step 1060 the LSM 520 is arranged not to request a desired couplingstate of the second electric machine 212. The LSM 520 outputs a requestfor a coupling state to the arbitrator 570 and may, as illustrated inFIG. 8 , output a ‘no-request’ signal 730 to the arbitrator 570, wherethe no-request signal 730 is indictive of the LSM 520 not requesting aspecific coupling state of the second electric traction motor 212 to theone or more wheels of the second axle 222. Thus, prior to time t₃ inFIG. 8 , the LSM 520 outputs the no-request signal 730 to the arbitrator570, or may output no signal to the arbitrator 570 in other embodiments.The arbitrator 570 may have a default coupling state. The defaultcoupling state may be coupled i.e. for the second electric tractionmotor 212 to be coupled to the torque path of the second axle 222. Thuswhen either a ‘no-request’ signal 730, or no request signal is receivedby the arbitrator 570, the arbitrator 570 may output a determinedcoupling request via the output means 340.

In some embodiments, the LSM 520 is arranged to output the couplingsignal 345, indicative of a request to couple the second electrictraction motor 212 to the at least one wheel of the second axle 222. Itwill be appreciated that the LSM 520 may, in some embodiments, requestthe default state of coupled when the speed signal 410 is indicative ofa low vehicle speed i.e. below the first LST 910.

In some embodiments, the LSM 520 may apply hysteresis to the speedsignal 410 to determine the coupling state. That is, the coupling stateof coupled may be determined for a vehicle speed greater than that atwhich the second electric traction motor 212 is determined to be coupledto the torque path via the second axle 222 i.e. above the first LST 910.Advantageously this assists in preventing ‘hunting’ or ‘flickering’between decoupled and coupled states as the speed of the vehicle variesaround (above and below) the first LST 910. Use of the second LST 920provides the hysteresis in some embodiments. As can be appreciated fromFIG. 8 , between t₃ and prior to time t₄ the vehicle accelerates from aminimum speed, such that the speed signal 410 exceed the first LST 910for a period of time prior to time t₄. As can be appreciated from thelower portion of FIG. 8 , the ‘no-request’ signal 730 is not outputimmediately upon the speed of the vehicle 100 exceeding the first LST910 i.e. coupled 750 is maintained.

Instead, in a region between the first and second LSTs 910, 920 thecoupling state of coupled 750 is maintained until the vehicle speedfalls exceeds the second LST 920 at time t₄. In step 1050, which isreached when the vehicle speed is between the first and second LSTs 910,920 the desired coupling state is determined in dependence on the speedsignal 410 in dependence on a last intersected of the first and secondLSTs 910, 920. Thus, prior to time t₄ when the speed signal 410 is belowthe second LST 920 the coupling state is determined in step 1050 ascoupled based on last-intersecting the first LST 910. Similarly,immediately prior to time t₃, when the speed signal 410 is above thefirst LST 920, the ‘no request’ output signal 730 is maintained as thelast intersected threshold is the second LST 920.

As can be appreciated from FIG. 8 , some embodiments of the LSM 520comprise a third LST 930. The coupling of the motor 212 is inhibited ifnot successfully coupled to the second torque path via the second axle222 when the vehicle speed 410 is equal to or less than the third LST930. The third LST 930 may correspond to a speed of, for example, 5kmh⁻¹ although it will be appreciated that other speeds may be selected.

In some embodiments, the LSM 520 is arranged to receive a signalindicative of a coupling status 470 of the second electric tractionmotor 212 to the at least one wheel of the axle 222. The signal 470reports whether the second electric traction motor 212 is successfullycoupled to the at least one wheel of the axle 222. In some situations,the coupling state may be determined as coupled and a correspondingrequest output by the controller 305. However for electrical and/ormechanical reasons it may not be possible, at least immediately, tocouple the motor 212 to the second torque path. For example, the secondclutch 219 may not have yet successfully engaged a drive output of themotor 212 to the axle 222. In particular, it may be difficult tosuccessfully couple the motor 212 when the vehicle is moving slowly orhas become stationary. Furthermore, attempted coupling of the motor 212to the axle may be increasingly noticeable, such as in the form of noiseand/or vibration, to occupants of the vehicle 100 at slow speeds and maypossibly cause damage if attempted whilst stationary. Use of the thirdLST 930 reduces such risks.

The LSM 520 in some embodiments determines a coupling inhibited state.The LSM 520 in some embodiments outputs a coupling inhibit signal 526 inthe coupling inhibited state when the speed signal 410 is indicative ofa vehicle speed equal to or below the third LST 930. The LSM 520 mayoutput the coupling inhibit signal 526 when the vehicle speed is belowthe third LST 930 and the coupling status signal 470 is indicative ofthe second electric traction motor 212 being decoupled from the secondaxle 222 i.e. successful coupling caused by the vehicle speed beingbelow the first LST 910 has not yet occurred.

In some embodiments, the LSM 520 may apply hysteresis to the speedsignal 410 to determine the coupling inhibited state. That is, thecoupling inhibited state may be determined for a vehicle speed greaterthan the third LST 930. Advantageously this assists in preventing‘hunting’ or ‘flickering’ between the decoupled and coupled states asthe speed of the vehicle varies around (above and below) the third LST930. Use of a fourth LST 950, as shown in FIG. 9 provides the hysteresisin some embodiments. The fourth LST 950 defines a maximum speed of acoupling inhibition region 940 defining the coupling inhibited state.The third and fourth LSTs 930, 950 act as described above with respectto the first and second LSTs 910, 92 and the speed signal 410.

Some embodiments of the invention comprise a fault management module(FMM) 530. The FMM 530 is arranged to determine a desired coupling stateof the second electric traction motor 212 to the at least one wheel (RL,RR) of the second axle 222 in dependence on detection or determinationof one or more faults associated with the vehicle 100. The couplingstate determined by the FMM 530 is selected to manage or mitigate faultsassociated with the vehicle 100. For example, the FMM 530 may receivethe temperature signal 420, wherein the temperature signal 420 isindicative of an invertor temperature associated with the secondelectric traction motor 212. In the event of the temperature signal 420indicating that the invertor has a high temperature (above apredetermined threshold), the FMM 530 is arranged to determine thecoupling state as decoupled in order to allow the second electrictraction motor 212 to be inactive thereby allowing the invertor to coolfor a period of time. In another example, the FMM 530 is arranged toreceive the coupling status signal 470 discussed above. The couplingstatus signal 470 may be indicative of a failure to decouple the secondelectric traction motor 212 to the axle. Therefore the FMM 530 maydetermine the coupling state as coupled in dependence thereon to reduceproblems associated with the problematic decoupled state. The FMM 530 isarranged to output a fault-derived coupling state request (FDCSR) signal535 in dependence on one more received signals indicative of fault stateassociated with the vehicle 100. The FDCSR signal 535 is indicative of acoupling state request determined by the FMM 530 in response to one ormore faults or undesirable conditions or parameters associated with thevehicle. The FDCSR signal 535 is received by the arbitrator 570 in someembodiments as shown in FIG. 4 .

In some embodiments, the FMM 530 is arranged to manage retries, i.e.further attempts, of changes in the coupling state of the secondelectric traction motor 212 in the presence of a failure to successfullychange the coupling state. In particular, in some embodiments, the FMM530 is arranged to control the output means 340 of the controller 305 tooutput a signal 345 indicative of a retry, i.e. to request a furtherattempt, of a change in the coupling state of the second electrictraction motor 212 as will be explained.

FIG. 11 illustrates a method 1200 according to an embodiment of theinvention. The method 1200 is a method of managing retries of a changein coupling state of the second electric traction motor 212.

In step 1210 the coupling state of the second electric traction motor212 is determined. The coupling state may be determined by one of themodules 510-560 and a consequent coupling state request signal receivedat the arbitrator 570, or by the arbitrator 570 such as in the case ofthe default coupling state in the absence of any requests from themodules 510-560.

In step 1220, a coupling state request signal 345 is output from thecontroller 305 via the output means 340 to request the determinedcoupling state. For example, the coupling state request may be a requestfor one of a coupled or decoupled state of the second electric tractionmotor 212 to the second axle 222.

In step 1230 the FMM 530 is arranged to determine whether a failure tochange the coupling state of the second electric traction motor 212 tothe second axle 222 has occurred. As discussed above, the couplingstatus signal 470 is indicative of the actual coupling status of thesecond electric traction motor 212 to the one or both wheels of thesecond axle 222. Therefore, the FMM 530 is able to determine, independence on the coupling status signal 470, whether the failure hasoccurred i.e. whether the actual coupling state reflects the requestedcoupling state. Step 1230 may be performed after a delay to allow achange in coupling state to be implemented, such as the second clutch219 being opened or closed. If the change in coupling state issuccessful the method returns to step 1210. If, however, the change wasnot successful i.e. a failure to change the coupling state of the secondelectric traction motor 212 has occurred as indicated by the couplingstatus signal 470, the method moves to step 1240.

In step 1240 a speed of the vehicle 100 is determined. Step 1240comprises receiving the speed signal 410 indicative of the speed of thevehicle 100. Controlling the output means 340 of the controller 305 tooutput the coupling signal 345 indicative of a retry of the change inthe coupling state is performed in dependence on the speed signal 410 aswill be explained.

In some embodiments, the FMM 530 is arranged to defer controlling theoutput means 340 to output the coupling signal 345 indicative of theretry of the change in the coupling state in dependence on the speedsignal 410 being indicative of the speed of the vehicle 100 being atleast a predetermined minimum speed. The predetermined minimum speed maybe, for example, a speed greater than substantially 0 kmh⁻¹. Otherpredetermined minimum speeds may be, for example, 5 kmh⁻¹ although itwill be appreciated that other minimum speeds may be selected.Advantageously, preventing a retry of the change in coupling state,particularly from changes from decoupled to coupled, at to too low avehicle speed may prevent the retry of the engagement of the secondelectric traction motor 212 with the axle being noticeable to occupantsof the vehicle 100. For example, such as (although not exclusively)where the second clutch 219 is a dog clutch, attempting the retry maycause noise and/or vibration at low vehicle speeds.

In some embodiments, the FMM 530 is arranged to defer controlling theoutput means 345 to output the coupling signal 345 indicative of theretry of the change in the coupling state in dependence on the speedsignal being indicative of the speed of the vehicle 100 being less thana maximum speed. The maximum speed may be, for example up to 50 kmh⁻¹ orup to 30 kmh⁻¹ or up to 20 kmh⁻¹ although other maximum speeds may bechosen. As noted above, in order to couple the second electric tractionmotor 212 to the second axle 222 it may be necessary to ‘spin-up’ oraccelerate the motor 212 to approximately the rotation speed of the axle222. Advantageously the maximum speed prevents or reduces energy used incoupling the motor 212 to the axle 222. Furthermore, changing from thedecoupled to the coupled state at vehicle speed below the maximum speedmay avoid attempting to couple the second electric traction motor 212 tothe axle during periods of large deacceleration i.e. during heavybraking or other slowing of the vehicle 100 when it may be difficult tomatch the rotation speed of the second electric traction motor 212 tothe axle 222. Thus the FMM 530 defers controlling the output means 340to output the coupling signal 345 indicative of the retry of the changein the coupling state in dependence on the speed signal 410 beingindicative of the speed of the vehicle being less than or equal to thepredetermined maximum speed.

In step 1250 the FMM 530, when it is determined that the speed of thevehicle 100 is either above the minimum speed or above the minimum speedand below the maximum speed considered in step 1240, the FMM 530 isarranged to output a signal 535 indicative of a request to retry thechange of coupling state. The signal 535 may be a further request forthe change in coupling state such as a request for one or the coupled ordecoupled state. The request may be received by the arbitrator 570 whichoutputs a corresponding request or signal 345 via the output means 340to cause the retry of the change in coupling state. Once the retry ofthe change has been requested the method returns to step 1230, where itis considered whether the retry has been successful.

In some embodiments, for every iteration of step 1250 a counter ismaintained to track a number of retries of the change in coupling state.The FMM 350 in some embodiments is arranged to attempt the retry up to apredetermined maximum number of times. That is, to perform step 1250 upto the maximum number of times. The maximum number of times may be 5, 3or 2 in some embodiments. Advantageously the maximum number of retriesmay prevent excessive numbers of retries to avoid damaging the system300, and/or reduces energy wasted ‘spinning-up’ the second electrictraction motor 212 to attempt further retries.

Some embodiments of the invention comprise an anti-fussiness module(AFM) 540. The AFM 540 is arranged to control changes in coupling stateof the second electric traction motor 212. In particular, the AFM 540 isarranged to control a timing of changes in the coupling state of thesecond electric traction motor 212. The AFM 540 may ensure that changesin the coupling state of the second electric traction motor 212 do notoccur too frequently i.e. that at least a predetermined period of timeis provided between changes in coupling state of the second electrictraction motor 212. The AFM 540 is illustrated in FIG. 4 as forming partof the arbitrator 570. It will, however, be realised that the AFM 540may be located elsewhere i.e. that other structures may be envisaged.

FIG. 12 illustrates a method 1300 according to an embodiment of theinvention. The method 1300 is a method of controlling changes incoupling state of the second electric traction motor 212 according to anembodiment of the invention. The method 1300 may be performed by the AFmodule 540.

In step 1310 of the method a coupling state of the second electrictraction motor 212 to the second torque path via the second axle 222 isdetermined. In other words, step 1310 comprises determining whether thesecond electric traction motor 212 is coupled to one or more wheels (RR,RL) of the second axle 222 of the vehicle 100. The determination isperformed in dependence on at least one attribute signal, such as thespeed signal 410 indicative of the speed of the vehicle 100, or thedriving mode signal 440. As described above, the coupling state of thesecond electric traction motor 212 may be determined by one of modules510, 520, 530, 550, 560 and a corresponding signal or request providedto the arbitrator 570. For example, the HSM 510 may provide a request todecouple the second electric traction motor 212 from the rear axle 222,whilst the FMM 530 may provide a request to couple the second electrictraction motor 212 to the rear axle 222. Thus requests for variouscoupling states may originate from different modules. Advantageously theAF module 540 is arranged to prevent frequent changes in coupling stateof the second electric traction motor 212 in order to avoid such changesbeing noticeable to occupants of the vehicle 100. Step 1310 may compriseone or more requests for a coupling state being received at thearbitrator 570 and, in particular, the AFM 540.

Step 1320 comprises determining whether a predetermined period of timehas elapsed since a last, or most recent previous, change in couplingstate of the second electric traction motor 212. The predeterminedperiod of time may be a period of time since a last request for a changein coupling state was output by the controller 305, or since asuccessful change in coupling state reported by the coupling statussignal 470. The predetermined period of time may be, for example, atleast 5 second, at least 10 seconds, at least 20 seconds or at least 30seconds. It will be appreciated that other periods of time may beenvisaged. If the predetermined period of time has elapsed the method1300 moves to step 1340.

If the predetermined period of time has not elapsed, the method moves tostep 1330 where the AFM 540 is arranged to wait i.e. to defercontrolling the output means 340 of the controller 305 to output thecoupling signal 345 indicative of the requested change in the couplingstate until expiry of the predetermined period of time since the lastchange in the coupling state. The AFM 540 may buffer incoming orreceived coupling state requests from the modules 510, 520, 530, 550,560 until expiry of the predetermined period of time, as it will beappreciated that the desired coupling state may be continuouslyre-evaluated during the predetermined period of time. Thus upon expiryof the predetermined period of time the coupling state may be determinedbased upon most recently-received coupling state requests rather thanimplementing a first-buffered request. Advantageously this ensures thatthe requested coupling state upon expiry of the predetermined period oftime reflects most recent attributes of the vehicle 100. Upon expiry ofthe predetermined period of time the method moves to step 1340.

In step 1340 the AF module 540 is arranged to control the output means340 of the controller 305 to output the coupling request signal 345 tocontrol coupling of the second electric traction motor 212 to the rearaxle 222. In some embodiments, the inhibit module is provided with asignal 575 indicative of an arbitrated coupling request, as will beexplained.

Some embodiments of the invention comprise an inhibit module 550. Theinhibit module 550 is arranged to control changes in coupling state ofthe second electric traction motor 212. In particular, the inhibitmodule 550 is arranged to allow for inhibition of one or more couplingstates of the second electric traction motor 212 to the rear axle 222.The inhibition of a coupling state prevents the inhibited coupling statebeing requested by the controller 305. The inhibit module 550 isillustrated in FIG. 4 as forming part of the arbitrator 570. It will,however, be realised that the inhibit module 550 may be locatedelsewhere i.e. that other structures may be envisaged.

The inhibit module 550 is arranged to receive the inhibit signal 460.The inhibit signal is indicative of one or more coupling states of thesecond electric traction motor 212 to the rear axle 222 which areprohibited or inhibited. The inhibit signal 460 may be indicative or oneof the coupled and decoupled states of the second electric tractionmotor 212 to the rear axle 222. Whilst the inhibit signal 460 is shownas one signal it will be appreciated that in other embodiments arespective signal may be provided for each of the coupled and decoupledcoupling states to indicate whether each state is inhibited. The inhibitmodule is arranged to output a coupling state inhibit signal 555 to thearbitrator which is indicative of a request for a coupling state asdescribed below. In particular, the coupling state inhibit signal 555 isindicative of a request for a coupling state when that coupling state isnot inhibited, thereby further indicating which coupling states are notinhibited.

FIG. 13 illustrates a method 1400 according to an embodiment of theinvention. The method 1400 is a method of controlling changes incoupling state of the second electric traction motor 212 according to anembodiment of the invention. The method 1400 may be performed by theinhibit module 550.

In step 1410 of the method a coupling state of the second electrictraction motor 212 to the second torque path via the second axle 222 isdetermined. In other words, step 1410 comprises determining whether thesecond electric traction motor 212 is coupled to one or more wheels (RR,RL) of the second axle 222 of the vehicle 100. The determination may beperformed in dependence on a determination of an expected amount ofpower required to spin-up the second electric traction motor 212 to thespeed of the rear axle as compared to an amount of power available fromthe traction battery 200. As described above, the coupling state of thesecond electric traction motor 212 may be determined by one of modules510, 520, 530, 560 and a corresponding signal or request provided to thearbitrator 570. For example, the HSM 510 may provide a request todecouple the second electric traction motor 212 from the rear axle 222,whilst the FMM 530 may provide a request to couple the second electrictraction motor 212 to the rear axle 222. Thus requests for variouscoupling states may originate from different modules. Advantageously theinhibit module 550 is arranged to prevent a coupling state of the secondelectric traction motor 212 being selected, such as in order to avoid astate associated with a fault. For example, when it is determined that afault exists which prevents the second electric traction motor 212 fromcoupling to the rear axle 222, the inhibit module 550 may inhibit thecoupled state to avoid the coupled state being selected. Similarly, insome embodiments, one or more coupling states may be inhibited dependentupon one or more of a power limit or capability of the traction battery200. For example, if it is determined that the capability of thetraction battery 200 to provide sufficient power to spin up the secondelectric traction motor 212 for coupling to the rear axle 222, thecoupled state may be inhibited in step 1410.

Step 1410 may comprise one or more requests for a coupling state beingreceived at the arbitrator 570 and, in particular, the inhibit module550. As explained below, the arbitrator 570 may determine an arbitratedcoupling state in dependence on the received requests.

In step 1420 it is determined whether the determined coupling state isinhibited. The determined coupling state may be the arbitrated couplingstate determined by the arbitrator 570. Step 1420 comprises comparingthe determined coupling state against the one or more inhibited couplingstates, such as where the coupled state is indicated as inhibited by theinhibit signal 460. Where the determined coupling state and the couplingstate indicated by the inhibit signal differ, or no coupling state isindicated as inhibited, the method moves to step 1430. If, however, thedetermined coupling state is indicated as inhibited by the inhibitsignal 460 the method returns to step 1410. In other words, the method1400 prevents a request for an inhibited coupling state being output instep 1430.

In step 1430 the inhibit module 550 is arranged to control the outputmeans 340 of the controller 305 to output the coupling request signal345 to control coupling of the second electric traction motor 212 to therear axle 222. That is, when the determined coupling state is notindicated as inhibited by the inhibit signal 460 a request for thedetermined coupling state is output by the controller 305.

Some embodiments of the invention comprise a driving mode module (DMM)560. The DMM 560 is arranged to determine a coupling state of the secondelectric traction motor 212 in dependence on a driving mode of thevehicle 100. The driving mode of the vehicle 100 is indicated by thedriving mode signal 440. The driving mode of the vehicle 100 may beselected by a driver or occupant of the vehicle 100, or may at least inpart be determined by a module or system of the vehicle 100, such as aterrain-response (TR) module which adaptively selects a driving modeincluding one or more settings of the vehicle and, in particular, apowertrain thereof such as a traction control mode thereof, for example.The driving mode may include driving selected settings, such as of thepowertrain, including a driving mode of the vehicle including one offorward, reverse or neutral in the case of an automatic gearbox or agear selection of a manual gearbox. The driving mode may include aselection of one of sport, normal or economy driving modes wheresettings of one or more of the engine, first and/or second electricmotors, suspension etc of the vehicle 100 may be adapted accordingly.Data indicative of the selected driving mode(s) is provided by thedriving mode signal.

FIG. 14 illustrates a method 1500 according to an embodiment of theinvention. The method 1300 is a method is a method of controllingchanges in coupling state of the second electric traction motor 212according to an embodiment of the invention. Some of the steps of themethod 1500 may be performed by the DMM 560.

In step 1510 an attribute-based coupling state of the second electrictraction motor 212 to the second axle 222 is determined. Thedetermination in step 1510 is performed in dependence on at least oneattribute signal, such as the speed signal 410 indicative of the speedof the vehicle. As described above, the coupling state of the secondelectric traction motor 212 may be determined by one of modules 510,520, 530, 560 and a corresponding signal or request provided to thearbitrator 570. For example, the HSM 510 may provide a request todecouple the second electric traction motor 212 from the rear axle 222,whilst the FMM 530 may provide a request to couple the second electrictraction motor 212 to the rear axle 222. Thus requests for variouscoupling states may originate from different modules. Step 1510 may beperformed by one of more the HSM 510, the LSM 520, and FMM 530. Step1510 may be performed in dependence on signals 515, 525, 535 except forthe driving mode signal 440. One of more signals indicative of thedetermined coupling states is provided to the arbitrator 570. The one ormore coupling states determined in step 1510 may be together referred toas first coupling states of the second electric traction motor 212.

In step 1520 a driving-mode-based coupling state of the second electrictraction motor 212 to the second axle 222 is determined. Step 1520 isdetermined in dependence on the driving mode signal 440.

In one example, the driving mode signal 440 may indicate a selecteddriving mode of the vehicle including selection of an efficiency-baseddriving mode. The efficiency-based driving mode is selected in order toprovide improved efficiency of the vehicle 100, i.e. reduced energyconsumption, such as the expense of performance of the vehicle 100. Theefficiency may be to improve consumption of fuel provided to the engine202 or to conserve electrical power consumed the motors 212, 216. Thedriving mode signal 440 is indicative of the selection of the efficiencybased driving mode which may be manually or automatically selected.Similarly, in another example, the driving mode signal may be indicativeof a neutral gear of the vehicle 100 being selected.

In dependence on the driving mode signal 440 the DMM 560 is arranged todetermine the coupling state of the second electric traction motor 212to the rear axle 222, such as one of coupled and decoupled. A signal 565indicative of the driving-mode-based coupling state is provided to thearbitrator 570. The driving-mode-based coupling state may be referred toas a second coupling state of the second electric traction motor 212.Thus the coupling state determined in step 1520 may be decoupled.

In another example, the driving mode signal 440 may indicate either adriver-selected or automatically-selected, such as by the terrainresponse module, driving mode such as requesting four-wheel drive of thevehicle 100 which requires coupling of the second electric tractionmotor 212 to provide power to the rear axle 222. Thus the coupling statemay be determined as coupled in step 1520.

In step 1530 it is determined whether the first and second couplingstates are the same i.e. equal. That is, whether the first couplingstate as one of coupled or decoupled is equal to the second couplingstate as one of coupled or decoupled. If the first and second couplingstates are equal then, method moves to step 1540. If, however, the firstand second coupling states differ then the method moves to step 1550.

In step 1540, the output means 340 is controlled to output the couplingsignal 345 indicative of the first and second coupling states i.e. oneof coupled or decoupled.

In step 1550, the output means 340 is controlled to output the couplingsignal 345 indicative of the first coupling state i.e. theattribute-based coupling state when the determined first and secondcoupling states differ. That is, the arbitrator 570 is arranged toallocate a higher priority to the first coupling state than the secondcoupling state. This is reflected in Table 1 below, as will beexplained, by the efficiency column being right-most such that couplingstates determined e.g. by the HSM 510 etc take precedence. Only when norequests are received from the other modules does arbitrated couplingstate independently follow the coupling state determined by the DMM 560.

FIG. 15 illustrates a coupling state determined by the DMM 560 accordingto some embodiments of the invention. In some embodiments, the DMM 560is arranged to determine the coupling state of the second electrictraction motor 212 in dependence on the driving mode signal 440 beingindicative of a mode, or gear selection, of the powertrain in particulara gearbox thereof, such as one of drive (D), neutral (N) and Reverse (R)i.e. a shifter position. As can be appreciated, the DMM 560 is arrangedto not to request 1630 a coupling state 1630 when the powertrain is notin neutral i.e. one of D or R is selected, or a gear of the gearbox isselected. In such a state the DMM 560 may output the no-request NRsignal. However, when N is selected 1620, as indicated by the drivingmode signal 440, the DMM 560 is arranged to output the coupling signal565 to request the decoupled state 1640. Thus the second electrictraction motor 212 is requested to be decoupled when N is selected.

As described above, some embodiments of the present invention comprisethe arbitrator 570. The arbitrator 570 is arranged to receive one ormore requests for coupling states of the second electric traction motor212 and to determine an overall or arbitrated coupling state of thesecond electric traction motor 212 to the second axle 222. Thearbitrator 570 is arranged to control the output means 340 of thecontroller 305 to output the coupling signal 345 indicative thereof. Thearbitrator 570 is arranged to allocate a predetermined precedence orpriority to at least some of the requests for coupling states fromdifferent modules. Table 1 below identifies requests for coupling staterequests received from the various modules of the controller 305, adefault coupling state i.e. in the absence of any other requests, and adetermined coupling state of the arbitrator 570.

TABLE 1 Effi- Arbi- FMM HSM LSM Neutral AWD AF ciency trated 530 510 520560 560 540 560 Default 570 D X X X X X X X D NR D X X X X X X D C D X XX X X X D C NR X X X X X X C NR NR C X X X X X C NR NR NR D X X X X D NRNR NR NR C X X X C NR NR NR NR NR NR C X C NR NR NR NR NR NR D X D NR NRNR NR NR NR NR C C C = Coupled, D = Decoupled, NR = No Request, X =Don't Care.

The arbitrator 570 is arranged to receive the FDCSR signal 535 from theFMM 530 at an input means thereof. It can be appreciated that thearbitrator 570 also receives a plurality of further coupling staterequest signals 515, 525, 565, i.e. from each of modules 510, 520, 560.Each coupling state request signal is indicative of a request for acoupling state of the second electric traction motor 212 to the at leastone wheel of the second axle 222.

Referring to FIG. 10 , which illustrates a method of determining thecoupling state in the presence of an FDCSR 535 from the FMM 530. Thearbitrator 570 is arranged to determine an arbitrated coupling state ofthe second electric traction motor 212 to the at least one wheel of thesecond axle 222 in dependence on the FDCSR signal 535 and the at leastone further coupling state request signal 515, 525, 565. The arbitrator570 is arranged to determine the arbitrated coupling state of the secondelectric traction motor 212 in precedence on the FDCSR signal 535 overthe at least one further coupling state request signal 515, 525, 565.

In FIG. 10 , in step 1110 the arbitrator 570 is arranged to receive theFDCSR signal 535 from the FMM 530. The FDCSR signal 535 is indicative ofa coupling state request as explained above. For example, the FDCSRsignal 535 is indicative of a request for one of a coupled or decoupledstate as indicated in Table 1.

In step 1120 the arbitrator 570 is arranged to receive any othercoupling state request signals i.e. from modules 510, 520, 525, 525,565, 560. It will be appreciated, as contemplated by Table 1, that atsome points in time no other coupling state request are received at thesame time as the FDCSR 535.

In step 1130, a coupling state of the second electric traction motor 212is determined in dependence on the FDCSR 535 and any other receivedcoupling state requests. As can be appreciated from Table 1 above, whenthe FDCSR signal 535 is indicative of the decoupled state (D), thearbitrator 570 is arranged to determine the arbitrated coupling state asdecoupled irrespective of a state of the further coupling state requestsignals 515, 525, 565. Thus the arbitrator 570 is arranged to determinethe coupling state of the electric machine 212 in precedence on theFDCSR signal 535 over the any further coupling state request signals. Inparticular, the arbitrator 570 is arranged to determine the decoupledstate of the second electric traction motor 212 in precedence on theFDCSR signal 535 being indicative of a request to decouple the secondelectric traction motor 212 over any further coupling state requests.

When the arbitrator 570 receives the high-speed coupling state request515, HSCSR, signal from the HSM 510 which is indicative of a request todisconnect (D) the second electric traction motor 212 from the secondaxle 222, as can be appreciated from Table 1, when no FDCSR 535 isreceived (NR) or when the FDCSR signal 535 is indicative of a coupled(C) request, the arbitrator 570 determines the arbitrated coupling stateof the second electric traction motor 212 as decoupled in dependence onthe request from the HSM 510 to, advantageously, protect the secondelectric traction motor 212 from excessive rotation speed. Thus, thedecoupled request from the HSM 510 takes precedence over the FDCSR 535when in contradiction.

In step 1140, the arbitrator 570 is arranged to output an arbitratedcoupling request signal 575 indicative of the arbitrated coupling stateto control coupling of the second electric traction motor 212 to the atleast one wheel (RL, RR) of the second axle 222. The arbitrated couplingrequest signal 575 is output via the output means 340 of the controller305 to control the coupling of the second electric traction motor 212.

FIG. 16 illustrates an overall operation of the system 300. Trace 1701represents an arbitrated coupling state request output by the controller305 as signal 345. Trace 1702 represents an actual coupling state of thesecond electric traction motor 212 to the at least one wheel (RL, RR) ofthe second axle 222. Trace 1703 is a connection inhibit signal and trace1704 is disconnection or decoupled state inhibit signal.

As can be appreciated, during period 1710 the DMM 560 determines thecoupling state is decoupled such as based on the driving mode signal 440being indicative of the efficiency-based driving mode. The couplingstate of coupled being inhibited, as indicated by 1703, does not have aneffect since the arbitrated coupling state is decoupled. During period1720 the DMM 560 determines the coupling state as coupled based on theIDD driving mode indicated by the driving mode signal 440. However, theconnection inhibit signal 1703 indicates that the coupled state isinhibited, thereby the actual state of the coupling is decoupled i.e.the coupling inhibited state takes precedence over the coupled staterequested by the DMM 560. However, during period 1730 once the coupledstate inhibit signal 1703 indicates that the coupled state is notinhibited, the coupled state is achieved. During period 1740 the coupledstate is requested as a default coupling state of the arbitrator 570,although partially during period 1740 the decoupled state is inhibitedas shown by trace 1704 although this does not affect the coupling stateduring period 1740 as coupled is still requested by the arbitrator 570.However, during period 1750 when the decoupled state requested by theHSM 510, due to decoupled still being inhibited, the coupled state ismaintained. Once the inhibition is cancelled during period 1760 thecoupling state of decoupled is requested by the arbitrator 570corresponding to the requested state of the HSM 510. During period 1760the LSM 520 requests the coupled state.

It will be appreciated that embodiments of the present invention can berealised in the form of hardware, software or a combination of hardwareand software. Any such software may be stored in the form of volatile ornon-volatile storage such as, for example, a storage device like a ROM,whether erasable or rewritable or not, or in the form of memory such as,for example, RAM, memory chips, device or integrated circuits or on anoptically or magnetically readable medium such as, for example, a CD,DVD, magnetic disk or magnetic tape. It will be appreciated that thestorage devices and storage media are embodiments of machine-readablestorage that are suitable for storing a program or programs that, whenexecuted, implement embodiments of the present invention. Accordingly,embodiments provide a program comprising code for implementing a systemor method as claimed in any preceding claim and a machine readablestorage storing such a program. Still further, embodiments of thepresent invention may be conveyed electronically via any medium such asa communication signal carried over a wired or wireless connection andembodiments suitably encompass the same.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of any foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed. The claims should not be construed to cover merely theforegoing embodiments, but also any embodiments which fall within thescope of the claims.

1. An electric machine control system for a vehicle, the electric machine control system comprising one or more controllers, wherein the vehicle comprises an electric machine arranged to be selectively coupleable to provide torque to at least one wheel of an axle of the vehicle, the control system comprising: an input to receive at least one attribute signal indicative of one or more attributes of the vehicle; an output arranged to output a coupling signal to control coupling of the electric machine to the at least one wheel of the axle; and a processor arranged to determine a coupling state of the electric machine to the at least one wheel of the axle in dependence on the at least one attribute signal, wherein the processor is arranged to control the output to output the coupling signal indicative of the determined coupling state, wherein the processor is arranged to defer controlling the output to output the coupling signal indicative of a change in the coupling state until expiry of a predetermined period of time since a last change in the coupling state indicated by the coupling signal.
 2. The control system of claim 1, wherein the predetermined period of time is 30 seconds.
 3. The control system of claim 1, wherein the predetermined period of time is 10 seconds.
 4. The control system of claim 1, wherein the change in the coupling state is from coupled to decoupled.
 5. The control system of claim 1, wherein the change in the coupling state is from decoupled to coupled.
 6. The control system of claim 1, wherein the at least one attribute signal comprises a speed signal indicative of a speed of the vehicle.
 7. The control system of claim 1, wherein the at least one attribute signal comprises a driving mode signal indicative of a driving mode of the vehicle.
 8. A powertrain comprising the system of claim
 1. 9. A vehicle comprising the control system of claim
 1. 10. The vehicle of claim 9, wherein the electric machine is arranged to be selectively coupleable to provide torque to at least one wheel of a first axle of the vehicle, and the vehicle comprises a second motive power source arranged to provide torque to at least one wheel of a second axle of the vehicle.
 11. The vehicle of claim 10, wherein the second motive power source comprises a second electric machine.
 12. A method of controlling coupling of an electric machine to provide torque to at least one wheel of an axle of a vehicle, the method comprising: receiving at least one attribute signal indicative of one or more attributes of the vehicle; determining a coupling state of the electric machine to the at least one wheel of the axle in dependence on the at least one attribute signal; and outputting a coupling signal indicative of the determined coupling state to control coupling of the electric machine to the at least one wheel of the axle, wherein the outputting comprises deferring outputting the coupling signal indicative of a change in the coupling state until expiry of a predetermined period of time since a last change in the coupling state indicated by the coupling signal.
 13. The method of claim 12, wherein the predetermined period of time is 30 seconds.
 14. The method of claim 12, wherein the change in the coupling state is from coupled to decoupled.
 15. The method of claim 12, wherein the change in the coupling state is from decoupled to coupled.
 16. The method of claim 12, wherein the at least one attribute signal comprises a speed signal indicative of a speed of the vehicle.
 17. The method of claim 12, wherein the at least one attribute signal comprises a driving mode signal indicative of a driving mode of the vehicle.
 18. A non-transitory, computer-readable memory storing computer software software which, when executed by a computer, is arranged to perform a method according to claim
 12. 19. The method of claim 12, wherein the predetermined period of time is 10 seconds. 